Wet-dry cycles
Clay-rich soils can swell as they absorb water and shrink as they dry. If moisture conditions differ from one side of a foundation to another, the support can become uneven.
Moisture sources
Rainfall, irrigation, roof runoff, grading, plumbing leaks, nearby trees, and drainage systems can all influence moisture around a foundation. No single factor should be assumed without looking at the property.
Soil is context, not a diagnosis
Knowing that expansive clay exists in the region does not prove that a particular crack or floor slope is foundation damage. The house still needs to be evaluated on its own conditions.
Local context: Educational statements about expansive clay, wet/dry soil movement, and common warning signs are consistent with current guidance published for Sugar Land and the greater Houston area. They are general context, not a diagnosis for any individual property.